1. Field of the Invention
The present invention is directed to a method and device for controlling or setting the illumination of an object for improving identification of an object feature in an image of the object, such as for recognizing a position of an electrical component or a substrate onto which the component is to be mounted at a designated position, or for quality control of components or substrates having a quality control marking thereon.
2. Description of the Prior Art
In the automatic equipping of printed circuit boards or ceramic substrates with components, particularly SMD (surface mounted device) components, the position of the components relative to the placement position on the printed circuit board is determined with means for position recognition before the placement. As used herein, the term "components" means all equippable articles including, for example shielding plates as well. Vision systems that employ a camera, for example a CCD camera, and an illumination means are generally employed as position recognition devices. A visual inspection of components ensues in analogously constructed apparatus for quality control in order to recognize faulty components and to remove them from the equipping process.
Centering marks applied on the substrates serve for the positional recognition of the substrates. Reject marks applied on the substrates are used for marking faulty substrates, so that these faulty substrates can be identified in the means for quality control and subsequently removed from the equipping process. A large variety of substrates (epoxy/fiberglass composites, plastic films, ceramics, resin-bonded paper) is employed. Different materials such as glossy or matte metals/metal oxides or plastic or lacquer coverings are also used for the markings. The optical surface properties of these various materials for substrates and markings vary extensively and thus so does the contrast between the marking and the substrate produced by an illumination. The illumination therefore should be selected such that useful structures (generally, the terminals of the components or the markings) are displayed with very high contrast relative to unwanted structures (for example, the body of the components or the substrate background).
To this end, methods are known wherein the components are illuminated from different angles with different intensities with the assistance of illumination devices that are subdivided into a number of lighting units with respectively separately controllable intensity control. The illumination devices thereby have a nearly constant spectral distribution.
For example, PCT Application WO 96/21343 discloses a method wherein, using a segmented ring lamp, the components are not uniformly illuminated from all angles; rather, no light is incident onto the structure from specific angles. Images of unwanted structures that emerge especially pronounced at these specific angles are thus suppressed.
Markings and substrates and/or components are respectively manufactured of materials that have optical properties such as absorption, metallic or diffuse reflection differing from one another in order to distinguish them from one another on the basis of their respectively different backscatter behavior upon illumination with an illumination means having nearly constant spectral distribution. The optical properties vary with the wavelength of the illuminating light, and thus certain material combinations for markings, substrates and/or components cannot be distinguished from one another in the illumination with nearly constant spectral distribution. A replacement of the illumination means dependent on the materials employed is very complicated and cannot be implemented due to the time losses associated therewith, which would prevent high equipping throughput.